1. Field of the Invention
The present invention relates generally to the fields of biology and chemistry, and bioanalytical instrumentation. In particular, the present invention is directed to composition and methods for use in sensing membrane potentials, especially in biological systems. Potentiometric optical probes enable researchers to perform membrane potential measurements in organelles and in cells that are too small to allow the use of microelectrodes. Moreover, in conjunction with imaging techniques, these probes can be employed to map variations in membrane potential across excitable cells and perfused organs with spatial resolution and sampling frequency that are difficult to achieve using microelectrodes.
2. Background of the Art
The plasma membrane of a cell typically has a transmembrane potential of approximately −70 mV (negative inside) as a consequence of K+, Na+ and Cl− concentration gradients that are maintained by active transport processes. Potentiometric probes offer an indirect method of detecting the translocation of these ions. Increases and decreases in membrane potential (referred to as membrane hyperpolarization and depolarization, respectively) play a central role in many physiological processes, including nerve-impulse propagation, muscle contraction, cell signaling and ion-channel gating (references 1-3). Potentiometric probes are important tools for studying these processes, and for cell-viability assessment. Potentiometric probes include the cationic or zwitterionic styryl dyes, the cationic carbocyanines and rhodamines, the anionic oxonols and hybrid oxonols and merocyanine 540 (references 4-8). The class of dye determines factors such as accumulation in cells, response mechanism and toxicity. Mechanisms for optical sensing of membrane potential have traditionally been divided into two classes: sensitive but slow redistribution of permanent ions from extracellular medium into the cell, and fast but small perturbation of relatively impermeable dyes attached to one face of the plasma membrane (references 2 and 3).
The bis-barbituric acid and thiobarbituric oxonols, often referred to as DiBAC and DiSBAC dyes respectively, form a family of spectrally distinct potentiometric probes with excitation maxima covering most of the range of visible wavelengths. DiBAC4(3) and DiSBAC2(3) have been the two most popular oxonol dyes for membrane potential measurement (references 9 and 11). These dyes enter depolarized cells where they bind to intracellular proteins or membranes and exhibit enhanced fluorescence and red spectral shifts. Increased depolarization results in more influx of the anionic dye and thus an increase in fluorescence. DiBAC4(3) reportedly has the highest voltage sensitivity. The long-wavelength DiSBAC2(3) has frequently been used in combination with the UV light-excitable Ca2+ indicators Indo-1 or Fura-2 for simultaneous measurements of membrane potential and Ca2+ concentrations. Interactions between anionic oxonols and the cationic K+-valinomycin complex complicate the use of this jonophore to calibrate potentiometric responses. DIBAC and DiSBAC dyes are excluded from mitochondria because of their overall negative charge, making them superior to carbocyanines for measuring plasma membrane potentials.
In general, DiBAC and DiSBAC dyes bearing longer alkyl chains had been proposed to have better properties for measuring membrane potentials (references 5 and 12). DiSBAC6(3) has been selected to use in a FRET-based membrane potential assay (reference 12). There are no reports on DiBAC1 and DiSBAC1 for measuring membrane potential.
It has been discovered that DiBAC1(3) and DiSBAC1(3) possess unexpected properties that can be used to measure membrane potentials with FLIPR and other fluorescence devices. Compared with other members of the DiBAC and DiSBAC family, DiBAC1(3) and DiSBAC1(3) give stronger signal and faster response besides its better water solubility.